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Suppose I have a time-traveling DeLorean (of all things), and I've used it to travel back to 1955. But then I ran out of juice. Fortunately, I just happen to know that a certain clock tower will be struck by lightning on a specific evening at a specific time, and I can use it to power a trip back to my own time.

I wonder if there's a flaw with this plan though: I'm assuming that lightning is deterministic. Lightning, being electrostatic discharge, seems like it could be subject to the laws of quantum mechanics. So, even if there were the exact same initial state as before the original lightning strike, whether it occurs or not is still a randomly determined outcome. I could be left there scratching my head as the moment passes and, oops, I can't (ahem) get back to the future.

To take another example, DNA mutations are subject to quantum randomness. If I travel back to the dawn of life, safely from space with zero impact on anything happening on Earth, am I still resetting all DNA mutations, completely changing the path of evolution, and wiping out humanity? Or on a lesser scale, if I travel back to before someone was born, am I resetting the quantum interactions determining their DNA, changing the fiber of who they are?

Are lightning or DNA formation actually influenced by quantum randomness? Are there any other ways quantum randomness could noticeably change history and throw my time traveling into disarray?

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    $\begingroup$ This is something I've always been curious about; if quantum randomization is 100% independent of the system, it seems like the simple (re)observation of it would collapse it differently; it seems like pure observation time travel would change history. $\endgroup$ – Nex Terren Oct 3 '16 at 22:19
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It depends on the time-travel model.

In the “closed timelike curve”, there is only one history and your actions in the past are as they always were. It’s impossible to change anything.

If it has multiple timelines, then naturally any quantum randomness will be different and independent in the new one. But, you can make arguments for things tending to be the same anyway, either outside of your lightcone or tending to try and keep the same values but being disturbed by the changes. It is a micro form of the concept that history is resistent to changes and small peturbations die out, unless some critical event sets a totally new course. If this is your model (generally safe for careful time travel) then expect quantum randomness to be unchanged or to help push back to the proper history. Generally, all the nearly-identical timelines tend to attact and re-enforce each other.

The situation for a single overwritable history (The Man who Folded Himself) is essentially the same. In fact, the people might think they have this case but actually have multiple “just as real” timelines (The End of Eternity). They had a different explaination for history wanting to stay in its course like a deeply cut river, and in a story with a true single time line you can do that.

For a timeline that’s explicitly reset to play forward again (Einstein’s Bridge), you expect quantum randomness to be distinct with less ability to handwave otherwise—you would need to explain how the apparent randomness comes from somewhere, which is a different formulation of quantum physics.

As I recall, in Thrice Upon a Time random changes were always happening spontainiously and this was the source of quantum randomness. They had a rather different view of multiple timelines like different boats going down the same river; changing the river’s flow affected subsequent boats.

So, if you had some specific effects in mind, you might be able to craft a universe to suit the story, either as careful details in the common models or something more novel.

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    $\begingroup$ The only good answer to a timetravel question like this one is an answer that starts with "It depends on the time-travel model." Also, an interesting tidbit: the equations for quantum mechanics are robust enough to permit consistency in time loops. Only when you bring in the non-deterministic interpretations like Copenhagen do you have any issues with determinacy at all. You only need those if you must have a classical world somewhere. Most modern theory is done on the assumption that the world is quantum, so no need for interpretations. $\endgroup$ – Cort Ammon Oct 4 '16 at 16:08
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Are there any other ways quantum randomness could noticeably change history and throw my time traveling into disarray?

Yes, there is the butterfly effect. Almost every real system is chaotic, i.e., that the exact outcome of the future is sensitive to tiny changes of the present. The classical example is that the flap of a butterfly’s wing affects the entire weather on the long run. You can only predict general behaviours, e.g., if you assume a thunderstorm as given (which it isn’t – due to the aforementioned butterfly), you can predict that there will be lightning, but not where and when exactly it is going to strike. This also applies to apparently stable systems such as a human: E.g., tiny perturbations may affect when exactly your heart beats (but you can be rather certain that your heart beats).

To take a more general point of view on this, the crucial question for you is how quick tiny perturbations (such as quantum effects or, more importantly, your presence) turn to affect the entire system. Unfortunately for you, a thunderstorm and in particular where and when lightning strikes is highly sensitive to tiny perturbations – at least as far as I can tell by my knowledge about lightning. Due to the nature of this effect, it cannot be measured and we can only deduce from models how sensitive a system is. So, if you arrive in the past a few minutes before lightning strikes, you may be lucky, otherwise lightning is probably going to strike somewhere else or even the entire thunderstorm may not happen.

(All of the above does of course not apply, if there is some mechanics specific to time travel that fixes the present.)

Also note that we do not know whether quantum effects or reality are stochastic or deterministic. It’s only that our best model for reality is stochastic (and we know about certain constraints of a possible deterministicity), but it’s impossible for us to tell whether reality is deterministic or not.

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The effect of quantum probability on the world

I'm going to take this as a thought experiment to the effect of: How much does non-determinism at the quantum level affect predictability at human scale?

The lightning strike

Imagine we return to before the lightning strike, and assume that we essentially rerun time from then, how different could or would it look from History 1?

The path of the lightning itself, and the place where it strikes, is set up (as far as I'm aware) by the distribution of temperatures and ions between the heavily charged cloud and the ground. So if we return to the very instant before the lightning strike, I would expect that strike to happen in exactly the same place; a lightning strike is a big thing set up by millions of ions.

If we step back a few seconds, then the picture is much less clear; we do not yet know exactly what triggers a lightning strike - the potential difference is not enough for a spontaneous spark. One possible trigger is cosmic ray showers creating an initial ionised path which then begins a cascade of electrons which amplify the ionisation and ultimately (sometimes) permit an arc. Would such a cosmic ray shower happen in the same way twice? Well, probably. The cosmic ray particle was already heading towards the Earth, though it is hard to make judgements about the determinism of its point of impact on the atmosphere; under the Copenhagen interpretation, we cannot talk about whether the particle was on a particular path beforehand.

Returning to a day or more before the strike I would intuitively say we are into the realms of the strike being unlikely to happen in the same place at the same time. The very actions of other lightning flashes (which are themselves unpredictable) within the storm structure will change the distribution of ions in and near the cloud. Even if the same cosmic ray particle induces a similar shower, the chances that it would invoke the same effect seem low.

Other uncertainties

A lot of the determinism that such a story relies on would largely work; people are large macroscopic beings driven by the actions of large macroscopic events, so really they are largely deterministic on short time scales.

However, even if people do largely the same things, the relative timing of those things may not be so deterministic; automobile incidents, lottery draws, even fertilisation are all sensitive to precise timing.

We intuitively understand this from our experience of life; chance meetings which would not have happened, chance tragedies, and so on. Nondeterminism even with such a small immediate effect leads to large differences even weeks or months later, let alone years.

Which history?

There is a slightly alarming lack of time-asymmetry in quantum physics. Things look largely the same backwards as forwards. Feynman diagrams codify this; an antiparticle is represented as a particle moving backward in time. Really all we have to tell us about the past are artefacts and memories which exist in the present; so the idea of going backward in time could be considered analogous to going forward into the future; it may be as indeterminate in either direction. Thus, it could be argued that you would only go back to one of many possible histories. From there, evolving time forward again would be extraordinarily unlikely to evolve to the same future, and you can wave goodbye to going back to the past again a second time.

Schrodinger's cat

This classic thought experiment was intended to highlight the oddness of connecting the macroscopic world directly to the quantum. It is not a reflection of reality, though; the mechanism which kills the cat is one that connects a macroscopic object to a quantum process, which is already set up to cause decoherence; the particle may be in a superposed state, but as soon as the vial is affected, the superposition has decayed and the cat is on one path or the other. We are still in the process of understanding the mechanisms and implications of the process of decoherence of a quantum state, particularly as we reach for tools to manipulate it in quantum computation.

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Your question predicate is irrelevant. Yes, the lightning is affected by quantum randomness, but it is an event that has already happened and that waveform has already collapsed (from your time traveling point of view). Unless you're somehow interfering (if that's even allowed under your rules of time travel) the bolt will happen the same -- quantum mechanics irrelevant to the answer.

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    $\begingroup$ Classical actions that interfere with chronology are quantum mechanical as well, just on a wildly different scale. It still counts. Mathematically, wave functions extend on forever, just to increasingly smaller degrees, so just being in 1955 will influence the lightning's wave function. The question is, how much does it influence it by? $\endgroup$ – user6511 Oct 3 '16 at 22:03
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    $\begingroup$ Hmm. I could buy that in a story, but I think it definitely depends on your mechanism of time travel and the entanglement over time. To me, you already were a part of the collapsed wave function by being in its light cone in the future, so you cannot change it. Personally, I prefer the Kage Baker theory: you can't (physically cannot) do anything that interferes with your subjective experience of future history. You can only act in the historic shadows. $\endgroup$ – SRM Oct 3 '16 at 23:10
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Random Number Generation.

Not pseudo-random number generation, but the real stuff. There's many ways to generate a truly random number, and I imagine the only ones that aren't hard to influence are ones that exist in relatively closed systems.

Systems that amplify radio noise (cosmic background radiation) will produce wildly different results if you just happen to walk near it.

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